1. Field of the Invention
The invention concerns a method and a system for reducing a jerk produced by the range shift of a CVT transmission with power division.
2. Description of the Related Art
Transmissions with continuously adjustable transmission ratio, especially belt-driven conical pulley transmissions with two conical disk pairs spanned by an endless torque-transmitting means, are increasingly utilized in motor vehicles because of the comfort advantages and fuel economy savings that can be achieved thereby. Because of the limited transmission ratio adjustment, which is made possible by the two conical disk pairs of the transmission ratio adjustment unit, also designated as a variator (variable speed drive), through opposite adjustment of the spacing between the conical disks of the conical disk pairs, the further development of such transmissions is thus intensively investigated so that the adjustment range of the variator can be used twice, in that it is carried out once in the one direction and then in the other direction.
FIG. 1 shows an exemplary CVT transmission with power division:
An engine 2 of a motor vehicle is connected with the input shaft 4 of a variator VAR containing two conical disk pairs spanned by an endless torque-transmitting means. The variator is connected with the output shaft 6 of the transmission by a first planetary gear set P1. A power division path of the transmission leads from the input shaft 4 through a clutch K2 and a second planetary gear set P2 to the output shaft 6.
More precisely, the input side of clutch K2 is non-rotatably connected with the input shaft 4 and the output side with the planet carrier of planetary gear set P1 and the ring gear of planetary gear set P2. The output shaft 6 is non-rotatably connected with the ring gear of the planetary gear set P1 and the planet carrier of planetary gear set P2. The output shaft of the variator is connected with the sun gear of the planetary gear set P1. The sun gear of the planetary gear set P2 is selectively fixed or freely rotatable through a shift clutch K1. To match a motor vehicle, the output shaft 6 is connected with the drive wheels 10 of the motor vehicle through a transmission ratio stage 8, wherein 12 symbolizes the elastic elements contained in the power train of the motor vehicle, which make the power train susceptible to vibration.
The variator VAR, and the shift clutches K1 and K2 are controlled by a control unit 14 with a microprocessor 16, a program memory 18, and a data memory 20. The control unit 14 has inputs 22, which are connected with sensors of the power train, for example rotational speed sensors, load sensors, temperature sensors, an accelerator pedal position sensor, a transmission selector lever position sensor, etc. The control unit 14 generates output signals at its outputs 24, in accordance with which the actuators of the variator VAR, and the shift clutches K1 and K2 are operated.
The construction and function of the described system are known and are therefore not described in detail.
FIG. 2 shows the overall transmission ratio iGES of the transmission as a function of the transmission ratio iVAR of the variator VAR, and the positions of the shift clutches K1 and K2.
In the first range of the transmission, while driving at high transmission ratios or at low speed, the shift clutch K2 is disengaged, and the shift clutch K1 engaged, so that the sun gear of the planetary gear set P2 is stationary, and the ring gear of the planetary gear set P2 rotates together with the planet carrier of the planetary gear set P1. The transmission runs in the non-power-division range, while its transmission ratio changes corresponding to the transmission ratio of the variator iVAR. When the variator reaches its longest possible transmission ratio (iVAR approximately=0.5), a shift occurs in the overall transmission ratio U, in which the shift clutch K1 is disengaged and the shift clutch K2 is engaged, so that a power-division operation takes place, which is carried out at higher speeds, and which leads to the longest possible overall transmission ratio iGES of the transmission of approximately 0.8. As shown, in that way by a spread of about 4 for the variator, a spread of about 7 for the overall transmission ratio can be achieved.
A problem that is encountered during a range shift of the transmission consists in the fact that rotating masses within the transmission, especially when shifting during rotational speed gradients take place, for example kick-down downshifts, require high acceleration output. That acceleration output is lost in the traction force, and thereby causes a shift jerk, or a shift jerking, which is detectable in the motor vehicle, and is relatively uncomfortable.
The invention is based on the object to avoid such jerks in a range shift.